In the documentation you can find almost all system variables and their functions:

Spoiler

NCount stands for the number of NPCs existing.BCount stands for the number of blocks existing.Player1scrX stands for the left-top x-position of the camera for player1.Player1scrY stands for the left-top y-position of the camera for player1.Player2scrX stands for the left-top x-position of the camera for player2.Player2scrY stands for the left-top y-position of the camera for player2.Playerhealth(Writable) stands for the current 1up number.CoinCount(Writable) stands for the coins number.Score(Writable) stands for the current score.StarCount stands for the current star number.Param1 stands for the first parameter transmitted from events.Param2 stands for the second parameter transmitted from events.BPlayer1Health stands for the 1-up number of the first player in battle mode.BPlayer2Health stands for the 1-up number of the first player in battle mode.EnableLighting(WriteOnly) stands for the state of the light system.Starcoincount stands for the number of acquired star coins.Disablesave(WriteOnly) stands for whether the player are forbidden to save.

Also we have found some undocumented variables (in daily tips and guessing them by ourselves)

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enablepause(Double) stands for the possibility to pauseshowhud(Double) stands for the HUD if its hidden or showndisablejump(Double) stands for the possibility to disable normal jumpdisablespinjump(Double) stands for the possibility to disable spin jump